Update

Influenza Activity — United States and Worldwide, May 19–September 28, 2019, and Composition of the 2020 Southern Hemisphere Influenza Vaccine

Scott Epperson, DVM; C. Todd Davis, PhD; Lynnette Brammer, MPH; Anwar Isa Abd Elal; Noreen Ajayi, MPH; John Barnes, PhD; Alicia P. Budd, MPH; Erin Burns, MA; Peter Daly, MPH; Vivien G. Dugan, PhD; Alicia M. Fry, MD; Yunho Jang, PhD; Sara Jo Johnson, MPH; Krista Kniss, MPH; Rebecca Kondor, PhD; Lisa A. Grohskopf, MD; Larisa Gubareva, PhD; Angiezel Merced-Morales, MPH; Wendy Sessions, MPH; James Stevens, PhD; David E. Wentworth, PhD; Xiyan Xu, MD; Daniel Jernigan, MD

Disclosures

Morbidity and Mortality Weekly Report. 2019;68(40):880-884. 

In This Article

Genetic and Antigenic Characterization of Influenza Viruses

CDC genetically characterized 867 influenza viruses submitted by U.S. and international laboratories during May 19–September 28, 2019, including 263 influenza A(H1N1)pdm09 viruses, 427 influenza A(H3N2) viruses, and 177 influenza B viruses. All A(H1N1)pdm09 viruses belonged to genetic subclade 6B.1A. Among 25 antigenically characterized A(H1N1)pdm09 viruses, 96% were similar** to the cell-culture propagated 2019–20 Northern Hemisphere vaccine virus component. The 427 influenza A(H3N2) viruses analyzed belonged to either clades 3C.2a (354; 83%) or 3C.3a (73; 17%) (Figure 2). Multiple subclades within the 3C.2a clade cocirculated with the majority of viruses belonging to subclade 3C.2a1, with regional differences in which subgroup of 3C.2a1 predominated. A(H3N2) viruses with a clade 3C.3a HA, which reemerged last season, continue to circulate in the WHO Region of the Americas. Among the 74 representative A(H3N2) viruses antigenically characterized, 70% were similar to the cell-culture propagated 2019–20 Northern Hemisphere vaccine virus component. Thus, although ferret antisera clearly distinguish antigenic differences between 3C.2a and 3C.3a viruses there is some cross-reactivity.

Figure 2.

Genetic characterization of U.S. and global viruses collected during May 19–September 28, 2019
HA clade/subclade of viruses by type (subtype) or lineage

All 21 of the influenza B/Yamagata lineage viruses analyzed belonged to clade Y3. All seven B/Yamagata lineage viruses antigenically characterized were similar to the cell culture–propagated 2019–20 Northern Hemisphere vaccine virus component. Multiple genetically and antigenically distinct B/Victoria lineage viruses cocirculated. Viruses with a two-amino acid deletion (162–163) in the HA protein belonged to subclade V1A.1, and viruses with a three-amino acid deletion (162–164) in the HA protein belonged to subclade V1A-3Del. Among the 156 influenza B/Victoria lineage viruses analyzed, the HA gene belonged to clade V1A (six viruses; 4%), subclade V1A.1 (37; 24%), or subclade V1A-3Del (113; 72%). Among the 53 B/Victoria lineage viruses antigenically characterized, the V1A.1 viruses were similar to the cell culture–propagated 2019–20 Northern Hemisphere vaccine component. Ferret antisera raised to recent V1A.1 viruses, however, had reduced reactivity with many viruses expressing V1A and V1A-3Del HA proteins indicating some antigenic differences between viruses in the different B/Victoria lineage subclades. Nevertheless, sera from humans vaccinated with a V1A.1 virus cross reacted well with V1A-3Del viruses.

** A virus is considered similar to a vaccine virus if it is well inhibited by ferret antisera raised against the cell culture– or egg culture–propagated reference virus representing the appropriate vaccine component for the specified season and hemisphere. The 2019–20 Northern Hemisphere vaccine components were A(H1N1)pdm09 subtype, A/Brisbane/02/2018-like (genetic group 6B.1A); A(H3N2) subtype, A/Kansas/14/2017-like (genetic group 3C.3a); B/Yamagata lineage, B/Phuket/3073/2013-like; and B/Victoria lineage, B/Colorado/06/2017-like (V1A.1) viruses.

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